Polyrhythms

Although the word polyrhythm technically means "many rhythms", it's most commonly used to describe the layering of multiple time signatures. For example, Steve Vai's The Attitude Song, in the main riff, has the guitars playing in 7/8 and the drums in 4/4 simultaneously. How could this work? What happens is this: 4/4 is the same as 8/8. So, when one bar of 7/8 is complete, the bar of 4/4 still has one eighth note remaining, so the "1" beat of the second bar of the 7/8 rhythm falls on the last eighth note of the 4/4 rhythm.
Now, after the second 7/8 bar has finished, the "1" beat of the third 7/8 bar will fall on the second-last eighth note in the second bar of the 4/4 rhythm. What you will notice from the example, in which guitar 1 plays in 7/8, and guitar 2 plays in 4/4, is that the "1" beat of the two rythms become increasingly displaced from one another by one eighth note, until eight bars of 7/8 are complete, at which point the "1" beat of the 7/8 rhythm syncronises with the "1" beat of the 4/4 rhythm again.

etc.
Notice the displacement? This causes a shifting effect that sounds cool when done properly. Often, when the two "1" beats re-synchronize, one of the instruments will change time the two instruments are in the same time.
Of course, you could layer any two, or three, time signatures you like. If you could find a musician who can count 128th notes, you could layer a 4/4 rhythm over a 127/128 rhythm, and they'd synchronize exactly every 128 bars! Now, some "homework": write and/or record music using some of these polyrhythmic layers:

Actually a Poly-rhythm must fit into the same bar. So in the case of a 3 over 4 or 3/4 over 4/4 rhythm, you would play triplets in 4/4 and another instrument would be accenting every 4th triplet note thus playing 8thnotes in 3/4 and effectively playing 3 beats over the other instruments 4 beats

Bazza79 wrote:
Actually a Poly-rhythm must fit into the same bar. So in the case of a 3 over 4 or 3/4 over 4/4 rhythm, you would play triplets in 4/4 and another instrument would be accenting every 4th triplet note thus playing 8thnotes in 3/4 and effectively playing 3 beats over the other instruments 4 beats

absolutely correct. that's the way polyrhythms are meant to be played. if you READ zappa's music/the research paper put forth by a music theorist regarding meshuggah, you'll know that this lesson only addresses simultaneously different time sigs, not polyrhythms as in, e.g., 5 against 3 or 7 gainst 5 rhythms

for further proof of my statement, i quote steve vai's statement at vai.com:
"A polyrhythm is just what it says. Two rhythms, or "feels", happening at the same time. Most people reading this have a good understanding of the basic triplet. This, in essence, is a polyrhythm. It's three 8th notes being played against two 8th notes"
so, if i played five 8th notes in the time which would generally have been required to play 3 8th notes (due to the tempo specified, eg at quart=120bpm, 3 8th notes would take 0.75 secs, but now i play 5 notes EVENLY in 0.75 secs) that would be a 5 against 3 polyrhythm.